Spacecraft, such as scientific and telecommunication satellites, include various instruments and devices that require deployment from the spacecraft in use, and/or require accurate positioning and realignment in use. For example, a satellite may comprise a deployable thermal radiator, an antenna on a boom or directly on the satellite structure that requires deployment, pointing and trimming in use, or a propulsion unit, mounted directly on the satellite or on a deployable boom, which requires accurate deployment and adjustment to provide spacecraft attitude control. It is therefore important that the driving mechanisms for such instruments and devices are capable of providing accurate control over their positioning for optimum operation, such as incremental rotational movement over small angles, or small deployment distance.